Assalaamu alaykum. My wife was raised by her maternal grandmother's female cousin. She is very close to her, to the point of considering her as her real grandmother. I wanted to know, please, if she can be considered as a Mahram (permanently unmarriageable kin) for me. May Allaah reward you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

A woman is considered Mahram (permanently unmarriageable) to a man for one of three reasons:

The Kuwaiti Fiqh Encyclopedia reads, “The reason for being a Mahram is due either to blood relation, or breastfeeding, or relationship by marriage.”

Therefore, the woman who raised your wife is not considered a Mahram to you.

For more benefit about the women who are Mahram to a man, please refer to fatwa 85868.

Allah knows best.